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From:   Uros Bizjak <ubizjak@gmail.com>
To:     linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Uros Bizjak <ubizjak@gmail.com>, "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>
Subject: [PATCH RESEND] fs/ext4: Use try_cmpxchg in ext4_update_bh_state
Date:   Wed,  2 Nov 2022 08:11:47 +0100
Message-Id: <20221102071147.6642-1-ubizjak@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
ext4_update_bh_state.  x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 fs/ext4/inode.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 2b5ef1b64249..162e39e2f59a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -780,11 +780,10 @@ static void ext4_update_bh_state(struct buffer_head *bh, unsigned long flags)
 	 * once we get rid of using bh as a container for mapping information
 	 * to pass to / from get_block functions, this can go away.
 	 */
+	old_state = READ_ONCE(bh->b_state);
 	do {
-		old_state = READ_ONCE(bh->b_state);
 		new_state = (old_state & ~EXT4_MAP_FLAGS) | flags;
-	} while (unlikely(
-		 cmpxchg(&bh->b_state, old_state, new_state) != old_state));
+	} while (unlikely(!try_cmpxchg(&bh->b_state, &old_state, new_state)));
 }
 
 static int _ext4_get_block(struct inode *inode, sector_t iblock,
-- 
2.37.3